The invention relates to an automation device, in which a plurality of spatially distributed functional units communicate with one another by means of a common transmission protocol. These functional units are in the form of field devices or control devices, corresponding to their automation functions.
In the field of measurement, open-loop and closed-loop control engineering, it has been customary for a relatively long time for a field device to be fed via a two-wire line and for measured values to be transmitted from this field device to a display device, and/or to a control installation, and/or for actuation values to be transmitted from a control installation to the field device. In this case, each measured value and/or actuation value is converted to a proportional direct current, which is superimposed on the feed direct current, in which case the direct current which represents the measured value or the actuation value may be a multiple of the feed direct current. By way of example, the feed current drawn by the field device is normally set to about 4 mA, and the dynamic range of the measured value or of the actuation value is mapped onto currents between 0 and 16 mA, so that it is possible to use the known 4 . . . 20 mA current loop.
More modern field devices are furthermore distinguished by universal characteristics, which can largely be adapted to the respective process. For this purpose, an alternating-current transmission path which can be operated bidirectionally is provided in parallel to the unidirectional direct-current transmission path, via which configuration data can be transmitted in the direction of the field device, and measured values and state data can be transmitted in the direction from the field device. The configuration data and the measured values as well as the state data are modulated, preferably frequency-modulated, onto an AC voltage.
In process control engineering, it is normal practice in the so-called field area to arrange in situ and to link field devices, that is to say measurement, actuation and display assemblies, in accordance with the predetermined safety conditions. These field devices have analog and digital interfaces for data transmission to one another. The data is in this case transmitted via the feed lines of the power supply, which is arranged in the console area. Controllers are also provided in the so-called console area for remote control and remote diagnosis of these field devices, the safety regulations relating to which are normally not very stringent.
The data transmission between the controllers in the console area and the field devices is carried out by superimposition of the known 20 mA current loops with the aid of FSK (frequency shift keying) modulation. In this case, two frequencies, which are associated with the binary states “0” and “1”, are transmitted in analog form, in frames.
The frame conditions for the FSK signal and the nature of the modulation are described in the “HART Physical Layer Specification Revision 7.1-Final” dated Jun. 20, 1990 (Rosemount Document No. D8900097; Revision B).
ASICs which have been designed specifically for this purpose, such as HT2012 from the company SMAR, are commercially available and in use for implementation of the FSK interface based on the HART protocol. These special circuits have the disadvantage that the functional scope is fixed and cannot be changed, and the lack of flexibility associated with this for adaptation to changing requirements is likewise disadvantageous.
Known modern automation devices are normally equipped with a processor unit, a so-called microcontroller, which is used for data processing in accordance with the stipulations, as a function of the automation task of the relevant functional unit.